The game designers use these gatekeeper bosses to ensure enough of the raid members did pass through the intended content progression before continuing deeper into the instance.

The term 'gear check' is sometimes used in a derogatory manner due to the fact that after surpassing the requirement, gear checks can become easy to the point of silliness (for example, with Tier 5 gear, the Curator is dead after 70 seconds or less).

Usually, the end boss fight of a raid instance is not designed as gear check, but rather requires high levels of skill even after beating it many times (possible exception: Prince Malchezzar, though this was probably not the designers' intent, as the "safe spots" were removed in patch 2.4).

For example Razorgore the Untamed (first boss of Blackwing Lair) requires a high amount of skill and coordination, but only a few key raid members are needed with good gear (on healers good gear may even be detrimental because of aggro issues). The second boss (Vaelastrasz the Corrupt), however, hits very hard and is on a three-minute timer. The whole raid must deliver very high DPS, several tanks with good gear must take turns in tanking him, and even with well-equipped tanks, the healers are under extreme pressure to keep them up. Most of the raid members must have spent a lot of time in Molten Core to gear up sufficiently in order to beat Vaelastrasz.

Trivia

The term can also be used to describe the minimum gear some guilds will accept when recruiting new members. Usually, this applies to raiding guilds, but can apply to any guild.This can also be called a "Gear Score"